It is now realized that solar electric power should be used whereever possible to minimize the use of fossil fuels which are in limited supply, to decrease atmospheric pollution, and to decrease capital and operating costs.
I. A conventional sodium-vapor street lamp, which is the most efficient (17.6%) light source may utilize 1000 watts of electric power between sundown and sunup for 10 hours during the night, thus requiring about 10 Kwh of electric energy. A solar panel may be exposed to available ambient sunlight for about 8 hrs a day. If the lamp is located in the north-eastern United States the incident solar power will vary from zero at night to a maximum of about 900 w/m.sup.2 in summer; with an average of about 500 w/m.sup.2 ; or 55.5%.
Presently available large area solar panels comprise amorphous silicon semiconductor coatings on stainless steel sheets. [4] These panels are rated at about 45 w/m.sup.2 at an efficiency of about 8%. A peak incident power of about 563 w/m.sup.2 corresponds to the midday sun in summer at a latitude of 40.degree.. At other times of the day and year, and on overcast days the power output is considerably less. Other factors are cost, presently about $6.00/w retail; and a useful life of 20 years. It is assumed the lamp is "on" to provide illumination from 7 PM to 5 AM at night, or for 10 hours per day; and "off" at other times. The sun is assumed to provide solar power to the panel from 9 AM to 5 PM, or for 8 hours per day. The energy required by a 1000 w lamp is then E.sub.1 =10,000 whrs. The energy produced by the conventional solar-panel is perhaps only about E.sub.2 =25 whrs/m.sup.2 for 8 hrs, or 200 whrs/m.sup.2. Hence the area of conventional solar panel needed to supply the 1000 w sodium vapor lamp is: EQU A.sub.a =E.sub.1 /E.sub.2 =10,000/200=50 m.sup.2 (1).
The peak rated power from this panel is 45 w/m.sup.2. The peak rated electric power output from the 50 m.sup.2 of conventional panel is: EQU P=50.times.45=2250w=2.25 kw (2).
The retail cost of these panels is: EQU C=($6./w).times.(2250 w)=$13,500. (3).
The rated life is 20 years, producing an average of about 55.5% of peak electric power, or about 1.5 kw electric power for 8 hrs. per day; hence the cost of the panel amortized over 20 years is: EQU c=($10,800.times.100).cent./(20.times.365.times.8 hrs.times.1.5 kw)=15.4.cent./kwhr (4).
This is without interest for amortizing the investment, but is a good first approximation. In a few years, the price of photovoltaics is expected to decrease to about $2.00/w, or to 5.1.cent.per Kwhr, which may be competitive with the cost of power from a conventional electric power source. In a system according to one embodiment of this invention, power generated by a solar panel is connected to a DC/AC solid state inverter and a two-way electric meter to the conventional electric power grid. The meter measures the kwhrs of electric power fed by the solar panel during the day to the grid; and the kwhrs supplied by the grid to the lamp during the night. The electric power Company is charged an agreed price for the energy supplied to the grid; and the electric power Company charges the owner of the System for any power used by the lamp. The costs of the power inputs and outputs may be designed to balance.
II. A solar street lamp illustrates an embodiment of the present invention. Instead of the convention solar panel and lamp above described, a quantum light/electric power converter is used, known as a LEPCON.TM. Panel. It comprises a plurality of submicron metal- insulator-metal tunnel junctions in metal strips deposited on glass forming arrays of antenna diodes [1]. The device is reversible, and is also employed as quantum electric/light power converter, known as ELCON.TM.. LEPCON.TM. or ELCON.TM. panels may be manufactured by a production device which employs a super submicron electron beam writer,termed a SUPERSEBTER.TM. [3]. These quantum device never deteriorate because they comprise metals and metal oxide deposited on a glass sheet. However, for comparison with the conventional panel above described, the same life of 20 years is assumed. The light/electric power conversion efficiency of the LEPCON.TM. panel is about 72%, or 9 times that of the conventional panel. The input solar light power is assumed to be 56% of peak 563 w/m.sup.2 ; or 312 w/m.sup.2. The solar energy input available during the sunlight hours of a day per m.sup.2 is: EQU E=8.times.312=2500 whrs =2.5 kwhrs/m.sup.2 -day (5).
The energy requirement for the ELCON.TM. lamp to supply the same light output as the sodium vapor 1000 watt lamp for 10 hours at night is: EQU E'.sub.1 =10 kwhrs/(72%/17.6%)=2.5 kwhrs (6).
The area of LEPCON.TM. panel required to supply this electric energy during 8 hours of sun per day is: EQU A.sub.b =(2.5 kwhrs/sun-day)/(2.5 kwhrs/sun day-m.sup.2)=1.0 m.sup.2( 7).
that is, a LEPCON.TM. panel of about 3'.times.3" is required to power this ELCON.TM. lamp.
Comparing the area A.sub.a from (1) of a conventional solar panel supplying a 1000 watt sodium vapor lamp, with the area A.sub.b of a LEPCON .TM. panel supplying an ELCON.TM. panel having the same light output, from (7), the area ratio is: EQU A.sub.a /A.sub.b =50/1.0=50 (8).
The retail selling price of the LEPCON.TM. panel is expected to be $500./m.sup.2. The cost of 1 m.sup.2 Lepcon.TM. panel is then: EQU C=1.times.$500.=$500. (9).
Electric energy produced by the LEPCON.TM. panel amortized over 20 years costs about: EQU c=(500..times.100)/(20.times.365.times.8.times.1.0)=0.86.cent./kwhr(9).
III. Alternatively, the light/electric power may be produced by a thin polymeric sheet known as LUMELOID.TM. [5]. It may have about the same efficiency, 72%, as the LEPCON.TM. panel. However, its life may be only one-half year. If the LUMELOID.TM. sheet provides 1.1 kwhrs solar/electric energy to an ELCON.TM. lamp, the area of the LUMELOID.TM. sheet is the same as in (6), 1.0 m.sup.2. The LUMELOID.TM. sheet is expected to see at retail for about $15./m.sup.2.
The cost of 1. m.sup.2 of LUMELOID.TM. sheet is: EQU c=(1 m.sup.2).times.($15..times.100.cent./m.sup.2)=1500.cent.,($15.)(10).
The cost in .cent./Kwhr, amortized over 1/2 year life is: EQU C=1500.cent./(1/2.times.365.times.8.times.1.kwhr)=1.0.cent./kwhr(11).
Table I Compares the efficiencies of various light sources with a lamp of the present invention:
TABLE 1 ______________________________________ 100% efficiency = 680 lumens/watt at 556 nm. [4] Type of Lamp Lumens Watts Efficiency ______________________________________ Incandescent 870 60 2.1% Sodium Vapor 120,000 1000 17.6% Quantum Electric/Light 120,000 200 72.% ______________________________________